Navigation systems assist users in precisely locating objects. For instance, navigation systems are used in industrial, aerospace, defense, and medical applications. In the medical field, navigation systems assist surgeons in precisely placing surgical instruments relative to a patient's anatomy.
Surgeries in which navigation systems are used include neurosurgery and orthopedic surgery. Often the instrument and the anatomy are tracked together with their relative movement shown on a display. The navigation system may display the instrument moving in conjunction with a preoperative image or an intraoperative image of the anatomy. Preoperative images are typically prepared by MRI or CT scans, while intraoperative images may be prepared using a fluoroscope, low level x-ray or any similar device. Alternatively, some systems are image-less in which the patient's anatomy is “painted” by a navigation probe and mathematically fitted to an anatomical model for display.
Navigation systems may employ light signals, sound waves, magnetic fields, RF signals, etc. in order to track the position and/or orientation of the instrument and anatomy. Optical navigation systems are widely used due to the accuracy of such systems.
Prior art optical navigation systems typically include one or more camera units that house one or more optical sensors (such as charge coupled devices or CCDs). The optical sensors detect light emitted from trackers attached to the instrument and the anatomy. Each tracker has a plurality of optical emitters such as light emitting diodes (LEDs) that periodically transmit light to the sensors to determine the position of the LEDs.
The positions of the LEDs on the instrument tracker correlate to the coordinates of a working end of the instrument relative to a camera coordinate system. The positions of the LEDs on the anatomy tracker(s) correlate to the coordinates of a target area of the anatomy in three-dimensional space relative to the camera coordinate system. Thus, the position and/or orientation of the working end of the instrument relative to the target area of the anatomy can be tracked and displayed.
Navigation systems can be used in a closed loop manner to control movement of surgical instruments. In these navigation systems both the instrument and the anatomy being treated are outfitted with trackers such that the navigation system can track their position and orientation. Information from the navigation system is then fed to a control system to control or guide movement of the instrument. In some cases, the instrument is held by a robot and the information is sent from the navigation system to a control system of the robot.
In order for the control system to quickly account for relative motion between the instrument and the anatomy being treated, the accuracy and speed of the navigation system must meet the desired tolerances of the procedure. For instance, tolerances associated with cementless knee implants may be very small to ensure adequate fit and function of the implant. Accordingly, the accuracy and speed of the navigation system may need to be greater than in more rough cutting procedures.
One of the limitations on accuracy and speed of optical navigation systems is that the system relies on the line-of-sight between the LEDs and the optical sensors of the camera unit. When the line-of-sight is broken, the system may not accurately determine the position and/or orientation of the instrument and anatomy being tracked. As a result, surgeries can encounter many starts and stops. For instance, during control of robotically assisted cutting, when the line-of-sight is broken, the cutting tool must be disabled until the line-of-sight is regained. This can cause significant delays and added cost to the procedure.
Another limitation on accuracy occurs when using active LEDs on the trackers. In such systems, the LEDs are often fired in sequence. In this case only the position of the actively fired LED is measured and known by the system, while the positions of the remaining, unmeasured LEDs are unknown. In these systems, the positions of the remaining, unmeasured LEDs are approximated. Approximations are usually based on linear velocity data extrapolated from the last known measured positions of the currently unmeasured LEDs. However, because the LEDs are fired in sequence, there can be a considerable lag between measurements of any one LED. This lag is increased with each additional tracker used in the system. Furthermore, this approximation does not take into account rotations of the trackers, resulting in further possible errors in position data for the trackers.
As a result, there is a need in the art for an optical navigation system that utilizes additional non-optically based data to improve tracking and provide a level of accuracy and speed with which to determine position and/or orientations of objects for precise surgical procedures such as robotically assisted surgical cutting.